1. Field of the Invention
The present invention relates to a method for improving the cold flow of hydrocarbon fuel oils.
2. Description of the Prior Art
Since the oil crisis, a variety of sources for fuel oils have been used and a ratio of amount of light crude oils has been reduced and therefore it is supposed that the use of heavy crude oils is in future more increased. On the other hand, the demand of middle distillate fuel oils tends to be increased in view of the regulation of exhaust of sulfur oxides. Therefore, if it is intended to obtain fuel oils as much as possible from heavy crude oils containing a large amount of paraffins having high molecular weight through fractional distillation, it is necessary to take out the distillate to fraction of high boiling points. As the result, the paraffin content having high molecular weight is increased in the distilled fuel oils.
In such fuel oils, crystals of paraffin are more apt to be precipitated and grown at a low temperature than in conventional fuel oils and the fluidity lowers. Furthermore, large paraffin crystal grains are formed even at a temperature at which the fluidity is maintained and a filter in the fuel supply stream and piping in diesel engine, etc. are plugged and the flowing of fuel oils is inhibited.
For solving these problems, a large number of cold flow improvers of fuel oils have been proposed, for example, condensation products of chlorinated paraffin and naphthalene (U.S. Pat. No.1,815,022), polyacrylates (U.S. Pat. No. 2,604,453), polyethylenes (U.S. Pat. No. 3,474,157), copolymers of ethylene and propylene (French Pat. No. 1,438,656) and copolymers of ethylene and vinyl acetate (U.S. Pat. No. 3,048,479) and the like.
When these cold flow improvers are added to fuel oils, they show excellent ability to lowering the pour point in a pour point test (JIS K 2269) but in many cases these cold flow improvers have substantially no effect in a cold filter plugging point test (abbreviated as CFPP hereinafter) by which the plugging of the filter in the fuel supply system at low temperatures is judged. The improvers that are effective with fuel oils containing a large amount of paraffin having high molecular weight, are few in number.
The pour point test cannot forecast the plugging of the filter in the fuel supply system due to paraffin crystal grains formed at a fairly higher temperature than the pouring point but the CFPP test serves to forecast this phenomenon and at present is widely used.